Rapid Voltammetric Determination of Traces of Zn2+ in 105 Excess of Ni2
Zn often appears in the Ni coatings deposited on brass, as a result of the brass detail dissolution in the low pH Ni baths before the beginning of the electrolysis: the deposition of the first Ni layer.
As known, the maintenance of the Zn2+ concentration in the Ni bath bellow 3 ppm can prevent Zn inclusions formation in the Ni coatings, which requires on-line Zn2+ determination. Nevertheless, spectral analytical methods such as AAS and ICP cannot be applied after Zn2+ extraction from the high concentrated matrix, because the huge Ni2+ concentration will provoke crystallization in the nebulizer area corrupting the analysis. The direct application of some voltammetric techniques also cannot yield any results, because of the very small difference of the E½ of the two species, which will cause peak overlapping. Some specific properties of the Ni2+ and Zn2+ ions however can be used to avoid the interferences. It is known that Zn2+ forms soluble hydroxyl complex in the presence of excess of OH-, while Ni2+ forms the very stable Ni(OH)2 precipitation product. Thus, KOH or NaOH can be applied as a supporting electrolyte, in which a well defined peak of Zn2+ is registered as shown in Fig. 1, while no peak of Ni2+will appear because of its precipitation.
The effect of the NaOH concentration applied as a supporting electrolyte, of the Zn2+ adsorption by the formed Ni(OH)2 precipitation product, and of the Ni2+/Zn2+ concentration ratio on the analytical characteristics of the Zn2+ determination are the subject of this study. The results are presented and discussed. It is demonstrated that the developed method consisting in adding of a small volume of Ni electroplating bath solution to the NaOH supporting electrolyte, followed by DPV determination can be applied for the on-line Zn2+ determination.